1. Field of the Invention
This invention relates to an optical head supporting apparatus for use in optical recording and reproducing systems which are to be used for an external memory unit of computers, a recording and reproducing unit for information such as, for example, audio signal and video signal, or the like.
2. Description of the Prior Art
With the recent trend of developments of computer and fast and extensive transmission means of information, non-volatile memories which are low in price, high in density, large in storage capacity as well as having high-speed data transportation capacity have been demanded Many of them use a magnetic disk system. However, there exist such problems as low recording density, high cost per one information unit, a large limitation of application environment and so on. Under such circumstances, the optical recording technique has come into the limelight as a technology to solve these problems.
When optical disk memories based on the optical recording technique are to be used, the technology for moving an optical head for recording information on the disk at a high speed is required, so that the optical head supporting apparatus thereof has been required to possess such properties as, for example, high accuracy, high rigidity and low friction. Therefore, many optical head supporting apparatus have been proposed recently.
Optical disks generally have a concentric or spiral recording track and a recording surface for data which is covered with a transparent substrate permeable to a laser beam. In order to record or reproduce data, it is necessary to apply a laser beam to the recording track by passing the beam thereof through the transparent substrate thereby to convergently irradiate it onto the recording surface. If a substantially converged parallel laser beam is entered directly into a transparent substrate, the laser beam will be scattered by surface defects or foreign matters, resulting in a deterioration in data quality to be recorded. In order to eliminate such an adverse effect on the data quality, convergent means such as, for example, an objective lens must be arranged in the neighborhood of the transparent substrate so that the laser beam can become a large laser spot on the surface of the transparent substrate and a converged laser spot on the recording surface thereof. That is, in recording and reproducing information using an optical disk, the objective lens is required to move up to the neighborhood of a recording track that is desired. Movement of the objective lens transversely to the recording track of an optical disk is called "seek movement". In addition, recording track of an optical disk has an eccentricity, which means that a beam spot on the recording surface has to follow the eccentricity. This is called "tracking movement". How to improve the speed for performing an accurate seek and tracking movements with respect to a specific recording track is the most essential problem to reduce the access time of an instrument. Further in addition, the optical disk has a surface deflection, so that in general the objective lens is moved in the direction perpendicular to the recording surface thereby to follow the deflection of the recording surface in order to form a converged beam spot thereon. This movement is called "focusing movement".
As a result, the objective lens is required to be moved two-dimensionally, a first direction which is either the seek or tracking direction parallel to the recording surface, and a second direction which is the focusing direction perpendicular thereto.
Conventional optical head supporting apparatus generally has guide means such as, for example, a guide shaft or guide surface arranged in the radial direction of an optical disk. A carriage is provided which moves linearly in accordance with the guide means. The carriage has a plurality of rolling members to carry out a relative movement with the guide means at a low friction. The carriage has an optical head at least including an objective lens. Seek movement is achieved by moving the carriage in the radial direction of an optical disk. However, since the mass becomes comparatively large as a result of such a structure so that a plurality of rolling members must be provided, the tracking movement is a slight movement of the objective lens only in the first direction. Also, the carriage is unmovable in the second direction, so that the focusing movement is achieved by moving only the objective lens in the second direction. Accordingly, the carriage is provided with an objective lens actuator for causing the objective lens to carry out tracking and focusing movements. These structures are disclosed by Japanese Patent Application Laid-Open No. 63-48620.
With such a structure, however, the following problems exist:
If foreign materials such as, for example, dust enter between the rolling members of the carriage and the guide means during seek movement, a large resistance to rolling will be developed. In general, such a rolling resistance is non-linear and has a large adverse effect on the control of seek movement.
Also, in order to carry out an access to a preferable recording track, the seek movement by the carriage and the tracking movement by the objective lens are required, each of which is a translation movement in the first direction so that their movements have duplicated degrees of freedom. Looked at from the viewpoint of a control system, the case where the carriage is to be moved and the case that the objective lens is to be moved are different in mode from each other, which means that each case requires providing an appropriate driving system and control system. In addition, the objective lens actuator contains a tracking mechanism and a focusing mechanism, each of which has vibration crosstalk as well as a resonance with a coarse seek movement of the carriage, so that the arrangement becomes a complex resonance system resulting in extreme difficulty of control. Further in addition, in order to hold a preferable recording track within the movable range of the tracking mechanism during seek movement, it is necessary to know the present position of the objective lens in the tracking direction, or to keep the objective lens always at a specific position in the movable range of the tracking mechanism, for example, at the middle point of the movable range thereof. This is because when the coarse seek movement is carried out supposing that the objective lens is positioned at the middle point of the movable range of the tracking mechanism, if the objective lens is positioned at the end of the movable range of the tracking mechanism by the tracking movement after completion of the coarse seek movement, the preferable recording track may be possibly be out of the movable range of the tracking mechanism.
Also, in order to move the carriage smoothly in the first direction, first of all, the surface accuracy of the contact area between the rolling members and the guide means will arise as a problem to be overcome, and then their relative position accuracy and the mounting accuracy of a plurality of rolling members are required to be maintained, which constitute a factor which increases the cost. In addition, durability and shock resistance are to be pointed out as a problem for the guide means and the rolling members for long term use.